This invention relates to a configuration of switched optical filters that is used to produce an optical wavelength division multiplexing (WDM) add/drop system.
In a long-haul, high capacity wavelength division multiplexing (WDM) optical network, the capability to remove and replace selective WDM channels at multiple add/drop nodes across the network is essential for a variety of value-added communications services including local access, interactive multimedia and wavelength leasing. Specifically, this capability requires dropping one or more optical channels at each add/drop network node from a multiplexed signal composed of a large number of such channels and, in some cases, substituting one or more optical channels that have the same carrier frequencies as the original channels.
WDM add/drop systems have been disclosed in the prior art using a complete frequency demultiplexing approach. Such systems have been constructed by pairing a 1xc3x97N channel demultiplexer, such as a so-called Dragone router, with an Nxc3x971 passive channel combiner, and operating on a subset of the N channel interconnections between these two devices to remove and replace selective channels (a system of this sort is disclosed in U.S. Pat. No. 5,526,153 issued to Glance on Jun. 11, 1996, hereinafter referred to as the Glance ""153 patent). In this configuration, an input signal composed of a plurality of multiplexed optical channels must first be fully demultiplexed to yield a sequence of individual optical channels. Thereafter, a subset of the demultiplexed channels is directed to circuitry where either a drop, add, or a drop and add of one or more of the individual channels can take place. Subsequently, the demultiplexed channels are directed to a multiplexer to produce a new plurality of multiplexed optical channels. Regardless of the number of optical channels to be added, dropped or dropped and added, the entire plurality of multiplexed optical channels must be demultiplexed and multiplexed. Although a large number of channels can be dropped and added using this configuration, it none-the-less presents several significant limitations.
When used in significant number over a long-haul WDM network, these paired demultiplexer/multiplexer add/drop nodes cause a substantial narrowing of the optical channel bandwidths. This narrowing can be further exacerbated by frequency misalignments between laser power sources and optical channel filters occurring, for example, due to temperature effects, polarization sensitivity and aging effects. Because the Dragone router allows some leakage of signals across adjacent channel paths, and introduces signal phase differences among the varying length channel paths, multipath interference is introduced. This interference can cause significant fluctuation in the power distributions of channels delivered through each demultiplexer/multiplexer pair (see, e.g., D. A. Fishman, D. G. Duff, J. A. Nagel, xe2x80x9cMeasurements and Simulation of Multipath Interference for 1.7-Gb/s Lightwave Transmission Systems Using Single and Multifrequency Lasers,xe2x80x9d Vol. 8, No. 6 J. of Lightwave Tech., June 1990). Signal power loss across each demultiplexer/multiplexer pair can be substantial, requiring insertion of costly optical amplifiers at an increased number of points in the network.
Channel bandwidth narrowing can be reduced by employing fiber-grating based selective filters that avoid demultiplexing and multiplexing the full set of WDM optical channels (see, e.g., H. Okayama et al., xe2x80x9cDynamic wavelength selective add/drop node comprising fibre gratings and optical switches,xe2x80x9d Vol. 33, No. 5 of Electronic Letters, pp. 403-404, Feb. 27, 1997). However, insertion losses and costs for these filters can be significant when used to drop and add more than a few channels. And signal dispersion can be problematic in fiber-grating based filter configurations optimized for adjacent channel rejections.
Cost and insertion loss are significantly reduced over the current art in a filter bypass system that consists of a series of switched filter-bypass devices at each add/drop node. The filters in these filter-bypass devices are individually designed to act upon one or more pre-selected channels in composite WDM signal, and the composite signal is switched to the input of a particular filter only when a corresponding channel is intended to be dropped and/or replaced. Otherwise, the composite signal bypasses the filter to reach subsequent filter-bypass devices in the series. Each device includes a switch capable of assuming several positions for routing the channels, and a filter for extracting (dropping) and inserting (adding) selected channels. Associated with each system is a controller coupled to the switch in each filter-bypass device for the purpose of selecting a desired routing position for that switch.
The inventive system provides several advantages over the prior art. Unnecessary signal processing of optical channels that are not being dropped or added is eliminated, thereby reducing the bandwidth narrowing, noise generation and power losses associated with prior art multiplexer/demultiplexer-based systems . In a preferred embodiment of the invention, the system employs thin-film filter technology to provide a significant improvement over fiber-grating based selective filter systems (see, e.g., H. Okayama et al., xe2x80x9cDynamic wavelength selective add/drop node comprising fibre gratings and optical switches,xe2x80x9d Vol. 33, No. 5 of Electronic Letters, p.p. 403-404, Feb. 27, 1997) by eliminating the need for optical circulators and by reducing the required number of optical switches. As a result substantial reductions in cost, dispersion and insertion loss are achieved.